It is known that photosensitive silver halide emulsions can be formed by precipitating silver halide grains in an aqueous dispersing medium comprising a binder, gelatin preferably being used as a binder.
The silver halide grains may be precipitated by a variety of conventional techniques. One common technique is a batch process commonly referred to as the double-jet precipitation process by which a silver salt solution in water and a halide salt solution in water are concurrently added into a reaction vessel containing the dispersing medium. Double-jet precipitation processes are described, for example, in GB 1,027,146, GB 1,302,405, U.S. Pat. Nos. 3,801,326, 4,046,376, 3,790,386, 3,897,935, 4,147,551, and 4,171,224. Precipitation of silver halide grains usually occurs in two distinct stages. In a first stage, the nucleation, formation of fine silver halide grain occurs. This is followed by a second stage, the growth stage, in which additional silver halide formed as a reaction product precipitates onto the initially formed silver halide grains, resulting in a growth of these silver halide grains. Batch double-jet precipitation processes are typically undertaken under conditions of rapid stirring of reactants in which the volume within the reaction vessel continuously increases during silver halide precipitation and soluble salts are formed in addition to the silver halide grains.
In order to avoid soluble salts in the emulsion layers of a photographic material from crystallizing out after coating and other photographic or mechanical disadvantages (stickiness, brittleness, etc.), the soluble salts formed during precipitation have to be removed.
The so-called ultrafiltration process is generally used for this purpose. The use of ultrafiltration for removing soluble salts and optimally concentrating silver halide emulsions is illustrated in Research Disclosures 10208 (1972), 13122 (1975), 13577 (1975) and 16351 (1977) and BE 818,237, and makes use of ultrafiltration membranes which can readily separate silver halide grains from the soluble salts and the liquid dispersing medium.
It is already known from U.S. Pat. Nos. 4,334,012 and 4,336,328 that ultrafiltration can be used during silver halide nuclei formation and silver halide grain growth of a silver halide emulsion with reduced variation in or, most preferably, constant volumes of the materials in the reaction vessel.
However, the last-mentioned process has the disadvantage that ultrafiltration during silver halide nuclei formation and grain growth may cause variability in the morphological and photographic characteristics of the silver halide emulsions. Therefore this process finds limitations as a general process for the preparation of silver halide emulsions.
DE 2,555,364 and U.S. Pat. No. 4,758,505 describe processes for silver halide emulsion preparation wherein the different stages of silver halide precipitation occur in different vessels.